.alert{
    width: 100%;
    height: 100%;
    position: fixed;
    left: 0;
    top: 0;
    background-color: rgba(0, 0, 0, .4)
}
.box{
    width: 80%;
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%,-50%);
    background-color: #fff;
}